A great deal of structural, chemical, functional and physiological information is available for hemoglobin, yet no wholly satisfactory model has been achieved for the molecular basis of respiration, the transport of oxygen by hemoglobin. In particular, the nature of the structural changes which take place on binding and release of ligands, and the way in which the properties of the heme are modified by incorporation into the globin, are still a matter of controversy. It is generally agreed that the initial conformational "trigger" involves subtle changes in the stereochemistry of the heme itself and of its immediate environment on binding ligands. These changes are transmitted via heme - globin interactions to the interfaces between the chains, ultimately leading to the change in quaternary structure and the expression of cooperativity. This proposal seeks to examine the heme - globin interactions directly by preparing horse hemoglobins with chemically modified hemes, altering both the peripheral substituents and the nature of the ligand. In such derivatives, the alterations in structure will be determined by the X-ray difference Fourier techniques of protein crystallography, and correlated with the alterations in functional properties observed by such techniques as ligand equilibria and (in collaboration with Prof. Quentin Gibson) stopped-flow and flash photolysis rapid kinetics, on several ligands. Our ultimate goal is a molecular mechanism which will account for the kinetic properties of hemoglobin, the way in which they differ from ligand to ligand and between the chains, and the way in which they are affected by modification of the heme.